The present invention generally relates to an automobile fuel control system and, more particularly, to an air-fuel control system for an internal combustion engine utilizing a closed-loop control operable during a particular engine operating condition to control the air-fuel ratio in dependence on the composition of exhaust gases.
Hitherto, there has been well known an automobile fuel control system wherein an air flowmeter disposed in an air intake passage is used to detect the flow of air sucked into the engine from time to time so that the amount of fuel to be injected can be controlled in dependence on the flow of the air so detected. The known system has an advantage in that, since the amount of air sucked into the engine can be detected directly, the air-fuel ratio of a combustible mixture can be controlled accurately.
However, the air flowmeter is delicate and expensive as is well known to those skilled in the art. Accordingly, when the air flowmeter is used in an automobile fuel control system in combination with a microcomputer, the result would be the increased price of the control system as a whole, and this goes against the recent demand for the accomplishment of an economy. In addition, if the fuel control system of the above described type is also used in combination with an exhaust gas recirculation (EGR) system for the minimization of the atmospheric pollutants, it is well recognized that the accurate air-fuel ratio control cannot be achieved unless the flow of exhaust gases being recirculated is correctly measured.
In view of the above, there has recently been proposed a so-called map control system wherein no air flowmeter is utilized and wherein various sensors such as, for example, an engine rotational speed sensor for detecting the engine rotational speed and a pressure sensor for detecting the absolute pressure within the fuel intake passage, which are generally used for the respective purposes other than for the fuel control, are concurrently utilized to detect an engine operating condition so that a predetermined amount of fuel pre-calculated in dependence on the engine operating condition so detected can be supplied. This fuel control system, i.e., the map control system, has developed into a so-called learning control system wherein change in operating performance of the engine with time (which change is hereinafter referred to as the "aging" of the engine) is taken into consideration in controlling the amount of fuel to be supplied.
As is well known to those skilled in the art, as the engine is operated for an extended period of time, some component part, for example, a valve seat forming a part of the fuel intake valve assembly is worn down to such an extent as to result in the change in timed relationship between the fuel intake valve and the exhaust valve. The wear of the valve seat described above is an example of the engine aging, and once this happens, for a given engine operating condition, the ratio between the amount of a dilution gas and that of an fresh air is adversely affected.
The Japanese Laid-open Patent Publication No. 55-96339, published July 22, 1980, discloses the use of a learning control technique wherein an oxygen sensor disposed in an engine exhaust system is used to determine whether or not the air-fuel ratio of the combustible mixture being supplied is accurately controlled to a stoichiometric value and wherein an amount of fuel appropriate for an individual engine operating condition which is generally determined by the suction pressure and the engine rotational speed is learned beforehand at an appropriate timing by sampling it so that the amount of fuel can be set to a value by the utilization of the learned value.
However, it has been found that, the prior art learning control system has a problem in that, since the oxygen sensor used to determine correctness or incorrectness of the control merely serves to determine whether the air-fuel ratio is lower than the stoichiometric value or whether it is higher than the stoichiometric value, the oxygen sensor even though it can determine that the air-fuel ratio has been enriched when the engine is operated under a high load condition in which the air-fuel is required to be enriched, fails to determine whether or not the amount of fuel being supplied during the high load engine operating condition has been correctly controlled.
Because of the presence of the above described problem, the prior art learning control system is so designed that, during the high load engine operating condition, an open-loop control is effected so as to enable a fixed map control precomputed on the basis of the suction pressure and the engine rotational speed.